Variable resistance key switch



Sept. 14, 1965 M. B. GREGORY 3,206,701

VARIABLE RESISTANCE KEY SWITCH Filed Oct. 29, 1962 5 Sheets-Sheet 1 Fig.4

INVENTOR. MARION B. GREGOR ATTO R N IY".

Sept. 14, 1965 M. B. GREGORY 3,206,701

VARIABLE RESISTANCE KEY SWITCH Filed Oct. '29, 1962 5 Sheets-Sheet 2 a)o) 25 w 33 @527 5: -25 U 1 i l x if; F1g-6 I I I I r 30 3/ 26 I 4 I, 3327 a? I m I'I m I r j i Fig.7 I I ai 30 31 33 2a 28 INVENTOR. MARION B.GREGORY,

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Sept. 14, 1965 M. B. GREGORY VARIABLE RESISTANCE KEY SWITCH 3Sheets-Sheet 3 Filed Oct. 29. 1962 Fig.14 42 47 4 5 Fig.15 4 0 fINVENTOR.

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Patented Sept. 14, 1965 3,206,701 VARIABLE RESISTANCE KEY SWITCH ManonB. Gregory, Cincinnati, Ohio, assignor to D. H. Baldwin Company, acorporation of Ohio Filed Oct. 29, 1962, Ser. No. 233,529 15 Claims.(Cl. 338-69) The invention relates to variable resistance key switchesfor use in electric musical instruments and the like. In musicalinstruments of the class in which oscillations are generatedcontinuously, and wherein tones are produced by connecting the sourcesof electric oscillations to voicing, output and sound reproducingdevices through corresponding key actuated switches, there is anelectrical surge when the key switch is opened or closed. As a resultthe tone comes on abruptly, and is generally accompanied by a thump atthe onset and sometimes by a click upon cessation, greatly detractingfrom the beauty and quality of the tone. It has been understood thatthis result can be overcome by the use of variable resistance keyswitches, such as the one described in the U.S. Patent No. 2,215,124 toKock et al. It is an object of this invention to provide improved formsof variable resistance key switches.

It is an object of the present invention to provide a variableresistance key switch which is simpler and less expensive tomanufacture.

It is an object of the present invention to provide a variableresistance key switch which will operate fully with a minimum ofmovement of the switch parts.

It is an object of the present invention to provide a variableresistance key switch with sufficiently low operating force to permitactuation of many switch elements per key, with an overall cost whichpermits such multiplication.

Another object of the present invention is to provide a switch whichwill yield a minimum rate of change in decibel level from first contactto full on position, while controlling the onset and cessation toneenvelopes to the end set forth above.

Another object of the invention is to provide a key switch as describedwhich will operate satisfactorily after as many as ten million cycles ofoperation.

These and other objects of the invention which will be describedhereinafter or will be apparent to those skilled in the art upon readingthis specification are ac complished by that construction andarrangement of parts of which certain exemplary embodiments will now bedescribed. Reference is made to the accompanying drawing wherein:

FIG. 1 is a perspective view of a plunger element and stop meanstherefor.

FIG. 2 is a plan view of the plate support means for one or a pluralityof resistance key switch gangs.

FIG. 3 is an end view of the support means as seen from the right sideof FIG. 2.

FIG. 4 is a plan view of an insulative panel showing five resistance keyswitch assemblies mounted thereon.

FIG. 5 is an end view of the insulative panel of FIG. 4 with the contactelements shown.

FIGS. 6, 7 and 8 show in elevation an insulative panel with a resistancekey switch assembly mounted thereon, illustrating the various positionsassumed by the contacting element.

FIG. 9 is a partial fragmentary view illustrating two resistance keyswitch assemblies operated by a single key.

FIG. 10 is a perspective view illustrating the tandem mounting of twoinsulative panels and their respective gang resistance key switchassemblies.

FIG. 11 is a plan view of another embodiment of the present inventionshowing an insulative panel and resistance key switch assemblies mountedthereon.

FIG. 12 is a plan view of still another embodiment of the presentinvention showing an insulative panel and resistance key switchassemblies mounted thereon.

FIGS. 13, 14 and 15 are cross-sectional views taken along the sectionline 1313 of FIGURE 12 illustrating the various positions assumed by thecontacting element.

FIG. 16 is a partial fragmentary view illustrating two resistance keyswitch assemblies, of the type illustrated in FIG. 12, in tandemmounting for operation by a single key.

FIGS. 17, 18 and 19 are cross-sectional views similar to FIGS. 13, 14and 15 illustrating the various positions assumed by the contactingelement when two strips of resistive material are used.

FIG. 20 is a perspective view of the spring element shown in FIG. 16.

Resistive switches of the type to be described are readily formed ingangs of varying length containing a considerable number of individualswitches. These switch gang structures can readily be used in tandem sothat the depression of one key will actuate a plurality of switches forthe purpose of feeding harmonically related oscillations into separateheaders, as in the organization of that type of electrical musicalinstrument which is set forth in U.S. Patent No. 2, 233,948 to Kock,issued March 4, 1941.

Briefly, in the practice of the invention a gang switch is provided witha unitary base plate or support which may be of sheet metal. The metalbase plate serves to hold the gang switches in place in the instrument,and to mount a series of plungers for generally vertical re ciprocation,these plungers adapted to be associated individually with the keys ofthe instrument, there being at least one switch in each gang. The baseplate is illustrated at 1 in FIGS. 2 and 3. It may be stiffened byturning up its edge portions as at 2 and 3 and may be of any lengthsuitable for convenient mounting in the instrument. The base plate isadapted to be supported either at its ends or upon a rail 4 as shown inFIG. 9 to which it may be attached by screws 5.

It will be understood by one skilled in the art that the base plate 1may support as many decks or gangs of resistive switch assemblies asrequired, and has a plurality of spaced perforations 6 for mounting suchdecks as will be more fully described hereinafter. The base plate isprovided with an upper support means for vertically reciprocatingplungers, which comprises a strip 7 of metal, plastic or other suitablematerial held in parallel spaced relationship to the base plate 1 bymeans of bolts 8 with suitable spacing sleeves 9 thereon. The strip 7has a plurality of spaced slots 10 adapted to serve as upper bearingsfor the reciprocating plungers. The base plate itself is provided with aseries of corresponding slots 11 (see FIG. 10) serving as lower bearingsfor the plungers.

FIG. 1 illustrates an exemplary plunger. The plunger, generallyindicated at 12, comprises a flat elongated strip of insulative materialof such width as to be slidable freely in the slots 10 and 11. The lowerend of the plunger 13 is of somewhat greater width than the slot 11. Theplunger is provided with notches 14 and 15 for retention of a washer orring-like structure 16. The ring 16 may be made of any suitableresilient insulative material; and as will be understood from FIG. 9will serve as a quietly operating stop for the plunger when it isactuated upwardly to its fullest extent. A second similar ring 19 isheld in the notches 17 and 18 in the plunger and will serve as a quietlyoperating stop when the plunger is in its lowermost position. Theplunger is also provided with angular notches 20 and 21 for theengagement of switch elements more fully described hereinafter. It willbe understood by one skilled in the art that the length of the plunger,the number of angular notches, and the height of the strip 7 53 from thebase plate 1 will be dependent upon the number of gangs of switchelements to be used.

FIGS. 4 and 5 illustrate a switch deck with a number of switchassemblies mounted thereon. The switch deck comprises an insulativepanel 22. The insulative panel has a plurality of perforations 23 whichcorrespond to the perforations 6 on the base plate 1. FIGS. 9 and 10illustrate how one or more of the insulative decks may be affixed inparallel spaced relationship to the base plate 1 by means of bolts 24extending through coaxial sets of perforations 6 and 23 and bearingspacing sleeves indicated at 25. The insulative deck 22 is provided witha plurality of spaced grommets 26 corresponding to the number of keyswitch assemblies to be mounted thereon. The rearmost edge of theinsulative deck is provided with a series of connectors 27 in alignmentwith the grommets 26 and affixed to the insulative panel or deck in anysuitable manner as for example by a punched out central portionextending through the insulative panel and bent over much like thegrommets.

Resistors 28 may be soldered between each adjacent pair of grommets 26and connectors 27. These resistors act as isolating resistors for theseveral generators and are connected respectively to the appropriategenerators through the connectors 27 A common electrical collector forall the switches of the panel 22, hereinafter referred to as the bus,extends the length of the insulative panel and is illustrated at 29. Thebus 29 is surrounded by a resistive material 30. The resistive materialmay be formed of any suitable conductive plastic material, as forexample vinyl, buna and hypalon, with the inclusion of carbon and/ orgraphite in their formulation. It has been found, however, thatexcellent results are achieved when the resistive material is made of aninorganic silicone elastomer. As will be noted in FIG. 5, the resistivematerial has a flat upper surface which slants downwardly toward thefront edge of the insulative deck at an angle of approximately 10 fromthe horizontal, that is, to the surface of the deck. The assembly of thecommon connector or bus and its resistive coating may be formed byextruding the plastic material about a wire or strip of metal.

A bar of insulative material, hereinafter referred to as the fulcrumbar, is located on the insulative panel parallel to and in spacedrelationship with the bus 29. The bus within the resistive material 30and the fulcrum bar 31 are held to the insulative panel 22 in spacedrelationship by means of clips 32. It will be noted from FIG. 5 that thefulcrum bar has a generally rectangular cross section.

A plurality of contact elements in the form of resilient wires 33 arelocated on the insulative deck 22, by soldering one end of each suchelement to one of the grommets 26. This will insure an electricalconnection between the contact element 33 and that resistor 23 which isconnected to the same grommet. The free end of each contact element willbe engaged in one of the angular notches in a plunger 12. For example,as shown in FIG. 10, when two decks of switch assemblies are employed,the ends of the contact elements will be engaged in the slanted notchesand 21 of the plunger.

The contact elements 33 may be made of any suitable resilient conductivematerial as for example straight round silver clad wires and the like.It has been found that wires of circular cross section are economical toproduce and require less dimensional control than do elements of othergeometric cross section.

FIGS. 6, 7 and 8 illustrate the various positions of the contact elementranging from its normal position shown in FIG. 6 to its fully depressedcondition shown in FIG. 8. As will be noted from FIG. 6 when the contactele ment is in its normal position it rests on the fulcrum bar 31 anddoes not contact the resistive material surrounding the bus. As thecontact element is depressed, the fulcrum bar acts as a fulcrum betweenthat end of the contact member soldered to the grommet 26 and theresistive material 30. The contact elements may have different naturalcurvatures; but through the combined action of the plungers and thefulcrum bar they are fully controlled. As a contact element isdepressed, it first contacts the uppermost corner of the resistivematerial 30. As the contact element is further depressed its initiallypoint contact with the resistive material becomes an area contact. Theremay at the outset be some slight compressive distortion of the edge ofthe resistive material; but the 10 included angle in the upper surfaceof the resistive material permits the wire to conform to its surface ascontact progresses.

It will be understood by one skilled in the art from FIGS. 6 through 9,that the insulative or fulcrum bar places a pre-load on the wire betweenthe fulcrum bar and the plunger, thus compensating to a large degree forpossible lack of straightness in the wire and thereby enhancinguniformity of the operation. Since the fulcrum bar now becomes thefulcrum or pivot bearing and is relatively close to the resistivematerial 30, the approach of the contacting element to the resistivematerial is slower in proportion to the travel of the plunger. When theplunger is fully depressed, however, the contacting element load thatbegins on the fulcrum bar is totally transferred to the resistivematerial 30. It will be understood also that the fulcrum bar assurescontact of the contacting element with the resistive material at thesame plunger position for all switches.

FIG. 9 is an exemplary showing of one embodiment of the switch of thepresent invention, illustrating the use of several insulative panelswith switch elements thereon and means for actuating the plunger. In amultiple switch assembly and a gang switch assembly, each of theplungers may be actuated by a corresponding playing key 34. The playingkey 34 is pivotally mounted on a fulcrum 35 and biased to anon-depressed position by the spring 36, causing the end of the key torest on a stop 37. The key is provided with a leaf spring 38 and acontrolling screw 39 to regulate its action on the plunger 12. It willbe clear to the one skilled in the art that as the key is depressed therear end of it will move upwardly in the direction of the arrow and theleaf spring 38 will contact the plunger 12 which in turn will actuatetwo contacting elements simultaneously. Other key constructions may, ofcourse, be used. For example, the key construction described in thecopending application entitled, Electronic Organ Construction, SerialNo. 831,984 in the name of Donald W. Elbrecht, filed August 6, 1959, maybe used. When such a key construction is used, the switch structure asillustrated in FIG. 9 will be inverted.

FIGS. 11 and 12 illustrate two additional embodiments of the switch ofthe present invention. The embodiment of FIG. 11 comprises an insulativepanel 4t) with a plurality of spaced grommets 41 and correspondingconnectors 42, with resistors 43 soldered therebetween. A resilient wirecontact element 44 is soldered to each of the grommets 41. The structurethus far described, is similar to that illustrated in FIG. 4. However,in this instance, a plate 45 is affixed to that side of the insulativepanel opposite the contact elements 44. The plate extends beyond theedge 46 of the insulative panel. A strip of resistive material 47 isheld in place on the outermost edge of the plate 45 by means of bentover tabs 48 which may be integral with the plate 45. The resistivestrip 47 is similar to the resistive strip 30 in FIG. 4 except that itdoes not carry within it a common electrical collector or bus. In thisinstance, the plate 45 acts as the bus. This embodiment furthereliminates the need for a fulcrum bar (as shown in FIG. 4 at 31), sincethe edge 46 of the insulative panel 40 serves as the fulcrum.

FIG. 12 illustrates a modified construction, and those parts which aresimilar to the structure of FIG. 11 are given like index numerals. Thestructure of FIG. 12 differs from that of FIG. 11 only in that theinsulative panel 40 is extended with the result that the edge 49 thereofabuts the resistive strip 47. The edge 49 of the insulative panel isprovided with a plurality of spaced notches 50, one for each contactelement. The rearmost edge 51 of each notch serves as the fulcrum forthe contact element, in the same way as does the edge 46 in FIG. 11. Anadvantage of the structure of FIG. 12 lies in the fact that the abutmentof the edge 49 with the resistive strip 47 reduces tolerance andalignment problems in the manu facturing process.

FIGS. 13, 14 and 15 are sectional views along the section line 14-14 ofFIG. 12 and illustrate the various positions of the contact element 44ranging from its normal position (FIG. 13) to its fully depressedposition (FIG. 15). It will be understood by one skilled in the art thatthe relationship of the rearmost edge 51 of the notch 50 of FIG. 12, andthe edge 46 of the insulative panel of FIG. 11 are such that FIGS. 13through 15 may be considered as showing the various positions of thecontact element 44 in the embodiment of FIG. 11 as well. It will benoted that that portion of the plate 45 hearing the resistive strip 47is angled slightly downwardly from the plane of the insulative panel sothat the contact surface of the resistive strip 47 will be properlyoriented with respect to the contact element 44 and the fulcrum edge 51.FIG. 13 illustrates the contact element 44 in its normal positionwherein it rests on the insulative panel edge 51. It is thereforemaintained out of contact with the resistive strip 47. As the outermostend of the contact element 44 is depressed, the contact element pivotsabout the fulcrum 51 and contacts the uppermost corner of the resistivestrip. As the contact element is further depressed (FIG. 15) its initialpoint contact with the resistive strip becomes an area contact and thecontact element load that began on the fulcrum edge 51 is totallytransferred to the resistive strip 47.

Thus it will be seen that the structures of FIGS. 11 and 12 accomplishthe purposes and advantages outlined above with respect to the structureof FIGS. 1 through 9. In addition, they afford a simpler structure bythe elimination of the fulcrum bar and the bus within the strip ofresistive material.

FIG. 16 is similar to FIG. 9 but illustrates the tandem mounting ofswitch assemblies of the type illustrated in FIG. 12. Insulative panels40 are mounted in parallel spaced relationship to a base plate 52 bymeans of bolts 53 with suitable spacing sleeves 54 thereon. As abovedescribed, the plate will have a series of slots therein serving aslower bearings for plungers, one of which is shown at 55. The plunger 55is provided with resilient insulative rings 56 serving as quietlyoperating stops for the plunger 55. As described above, the plunger 55engages the contact elements 44 and causes them to contact the resistivestrips 47 when the plunger is raised by a playing key assembly (notshown).

FIGS. 16 and 20 illustrate a structure for providing support for theupper end of the plunger 55, while at the same time biasing the plungerto its key-off posi tion. A plate 57, of angle iron cross section, issupported in spaced relationship with the base 52 at the upper ends ofthose bolt spacer assemblies 53, 54 nearest the plungers. The plate 57serves as a support for a strip of re silient material 58 having aplurality of outwardly eX- tending arms 59, each of which engages theupper end of a plunger 55. As will be noted in FIG. 20, the upper end ofeach plunger 55 is provided with a centrally located notch 60, and theoutwardmost portion of each resilient arm 59 is provided with inwardlyextending notches 61 and 62. The cooperation of the notches 61 and 62 inthe resilient arm and the notch in the plunger 55 forms aninter-engagement between the arm and the plunger such that the arm willnot only bias the plunger to its key-01f position, but also willmaintain the plunger in its proper vertical orientation. The strip 58will be held in place on the plate 57 by the passage of the bolt 53therethrough and the nut 63.

FIGS. 17 through 19 are similar to FIGS. 13 through 15, and differ onlyin that two respective strips 64 and 65 are substituted for the singlerestrictive strip 47. As shown in FIG. 17, the contact element 44, whenin its normal position, rests on the fulcrum edge 51 of the insulativepanel 40, out of contact with the resistive stri s 64 and 65. As theelement 44 is moved downwardly (FIG. 18) its initial point of contactwill be with the highest corner of the resistive strip 65. As thecontact element continues in its downward motion, it will contact thesurface of the resistive strip 65 and then the uppermost corner of theresistive strip 64. In its fully depressed position (FIG. 19) theelement 44 will contact the surfaces of both resistive strips. It hasbeen found that this structure gives a better starting characteristicfor the tone created. This structure exends the range of reisstance fromthe initial point of contact of the first strip to the fully depressedposition. The resistance letdown is more gradual as is the build up ofthe tone. It has also been found that the contact surface of the strip65 may be scored or roughened by any suitable means which tends to makethe initial resistance as high as desirable. The two strips, if desired,may have different specific resistivities.

Modifications may be made in the invention without departing from thespirit of it. For example, when a direct contact switch is desired, theabove described embodiments may be provided with a bus without resistivematerial. Reference is made to the copending application in the name ofWalter Munch, Jr., entitled Switching System for Electrical Organs,Serial No. 233,560 filed October 29, 1962. The coaction of the contactelements and a fulcrum bar or fulcrum edge of an insulative panel willcompensate to a large degree for possible lack of straightness in thewire contact elements, will assure contact between the bus and wireelements at the same plunger position for all switches, and will thusenhance uniformity of operation.

The invention having been described in certain exemplary embodiments,what is claimed as new and desired to be secured by Letters Patent is:

1. In a musical instrument wherein each tone is controlled by anelectric switch actuated by a playing key, a switch assembly for thepurpose described comprising a first elongated stationary contactelement, a series of second movable contact elements, said first andsecond contact elements mounted on an insulative base substantially atright angles to each other, said second contact elements being resilientwire-like elements one end of which, remote from said first contactelement is affixed to said base, means in connection with said base nearsaid first contact element to act as a fulcrum for said second contactelements to determine the time of contact of said second contactelements with said first contact element as the outer ends of saidsecond contact elements are drawn downwardly, said firs-t contactelement having a surface of resilient conductive substance which surfacelies aslant to the plane of the outer ends of said second movablecontact elements when the latter are first brought into contact withsaid first contact element, whereby further movement of said secondcontact elements will cause them to contact said first contact elementalong a line of increasing length transversely of said first contactelement.

2. The structure claimed in claim 1 wherein said first contact elementcomprises a metallic strand encased in a body of resistive substance.

3. The structure claimed in claim 1 wherein said first contact elementcomprises a metal plate attached to said insulative base near one edgethereof, said plate bearing a strip of resistive substance in a positionto be contacted by said second contact element.

4. The structure claimed in claim 3 wherein said plate holds said stripof resistive substance in substantial abutment with said edge of saidinsulative base but below the top surface thereof and wherein said edgeof said base is notched to provide said fulcrum means.

5. The structure claimed in claim 3 wherein said plate supports twostrips of resistive substance in side-by-side relationship, and in whichthe respective positions of the surfaces of said strips is such thatthey will be contacted seriatim by said second contact elements.

6. In a musical instrument wherein each tone is controlled by anelectrical switch actuated by a playing key, a switch for the purposedescribed comprising a first stationary contact element, a secondmovable contact element, said first and second contact elements mountedon an insulative base at right angles to each other, insulative means inconnection with said base to serve as a fulcrum for said second contact,resistive means surrounding said first contact and having a flat contactsurface angled slightly downwardly and away from the direction of motionof said second contact, said second contact being operatively connectedwith a playing key whereby in closing said switch said second contactcoacts .'ith a continuously increasing area of said contact surface toclose said switch first through a high resistance and then through acontinuously lowering resistance as said key is depressed.

7. In a musical instrument wherein each tone is controlled by anelectrical switch actuated by a playing key, a switch for the purposedescribed comprising an insulative base, a stationary contact means, aresistive means associated with said stationary contact means, means formounting said stationary Contact and said resistive meansnear andparallel to one edge of said base, a connector mounted on the oppositeedge of said base, an insulative bar afiixed to said base between saidconnector and said stationary contact, said insulative bar being nearand parallel to said stationary contact, a movable contact element, oneend of said movable contact element fixedly mounted to said base, meansfor connecting said fixed end to said connector, said resistive meanshaving a flat contacting surface angled slightly downwardly and awayfrom said insulative bar, means actuable by said playing key engagingthe free end of said movable contact causing said contact to coact withsaid insulative bar and with resistive means to establish a circuit whensaid key is depressed, whereby in closing said switch said insulativebar acts as a fulcrum about which said movable contact pivots to contactsaid resistive means, the area of contact between said movable contactand said flat contact surface increasing to close said switch firstthrough a high resistance and thence through a continuously loweringresistance upon progressive key depression.

8. The structure claimed in claim 6 wherein said resistive means is anextruded member surrounding said stationary contact and is composed of amaterial chosen from a class consisting of an elastomer containingconductive material.

til

9. The structure claimed in claim 8 wherein said movable contact is aresilient wire member.

10. The structure claimed in claim 9 wherein said means for joining saidfixed end of said movable contact element to said connector is anisolative resistance.

11. The structure claimed in claim 9 wherein said angularity of saidfiat contact surface of the resistive means is of the order of 10.

12. The structure claimed in claim 11 in which said resistive substanceis a silicone elastomer containing carbon.

13. In a musical instrument wherein each tone is controlled by anelectrical switch actuated by a playing key, a switch for the purposedescribed comprising an insulative base member, an elongated stationarycontact element mounted thereon and having a surface at an anglethereto, an elongated movable contact element having a free end and anend fixed to said base and at right angles to said stationary element,insulative fulcrum means on said base between the fixed end of saidmovable element and said stationary contact, means connecting said freeend of said movable contact to said key, whereby upon depression of saidkey, said movable contact is caused to meet said stationary contactsubstantially at a point thereon, thereafter contacting an increasingarea as said key is further depressed.

14. The combination claimed in claim 13, wherein said fulcrum meanscomprises an edge of said base member.

15. In combination a metal plate having a plurality of aligned slitsadjacent one edge thereof, guide means having a similar plurality ofaligned slits supported in spaced relationship from said plate, aplurality of flat, plunger-like elements operating respectively in theslits of said plate and guide means, at least one deck comprising aninsulative panel supported in spaced relationship from said plate, wireelements each having mounting means adjacent one edge of said deck, aresistive element comprising a metal bus and a coating thereon ofresilient substance containing conductive material, said bus beingsupported on said deck adjacent the other edge thereof, insulativefulcrum means associated on said deck and lying in parallelism to saidbus, said wire elements extending beyond the last mentioned edge of saiddeck and engaging in slits in said plunger-like elements.

References Qited by the Examiner UNITED STATES PATENTS 2,558,659 6/51Mork 338-69 2,559,077 7/51 Johnson et al.

2,931,877 4/60 Henley 338-114 X 2,959,693 11/60 Meyer 33869 X o RICHARDM. WOOD, Primary Examiner.

15. IN COMBINATION A METAL PLATE HAVING A PLURALITY OF ALIGNED SLITSADAJCENT ONE EDGE THEREOF, GUIDE MEANS HAVING A SIMILAR PLURALITY OFALIGNED SLITS SUPPORTED IN SPACED RELATIONSHIP FROM SAID PLATE, APLURALTIY OF FLAT, PLUNGER-LIKE ELEMENTS OPERATING RESPECTIVELY IN THESLITS OF SAID PLATE AND GUIDE MEANS, AT LEAST ONE DECK COMPRISING AN INSULATIVE PANEL SUPPORTED IN SPACED RELATIONSHIP FROM SAID PLATE, WIREELEMENTS EACH HAVING MOUNTING MEANS ADJACENT ONE EDGE OF SAID DECK, ARESISTIVE ELEMENT COMPRISING A METAL BUS AND A COATING THEREON OFRESILIENT SUBSTANCE CONTAINING CONDUCTIVE MATERIAL, SAID BUS BEINGSUPPORTED ON SAID DECK ADJACENT THE OTHER EDGE THEREOF, INSULATIVEFULCRUM MEANS ASSOCIATED ON SAID DECK AND LYING IN PARALLELISM TO SAIDBUS, SAID WIRE ELEMENTS EXTENDING BEYOND THE LAST MENTIONED EDGE OF SAIDDECK AND ENGAGING IN SLITS IN SAID PLUNGER-LIKE ELEMENTS.